Generally, in an internal combustion engine, high voltage, or sparking voltage, is generated by an ignition coil of the engine and is sequentially distributed to spark plugs associated with engine cylinders via a distributor. The high voltage causes the spark plugs to ignite a combustible gas mixture supplied to the combustion chambers.
It is desirable in an internal combustion engine to use air-to-fuel (A/F) ratios having more air than is chemically required by stoichiometry for complete burning. When a surplus of air exists in the combustion chamber, many pollutants, such as NOx (oxides of NO), CO, and HC are effectively reduced in the exhaust emissions. Unfortunately, it is very difficult to operate an engine with a surplus of air in the combustion chamber(s) associated with the engine cylinders because the engine will exhibit poor response to transient loading on the engine. Under a particular loading condition, the A/F ratio of the engine may be adjusted for acceptable "lean burn" operation, i.e., low emissions and no misfire. However, as the load changes, the A/F ratio required to prevent misfire also changes. If the A/F ratio is too low, then excessive emissions occur. If the A/F ratio is too high, then the engine misfires.
A misfire occurs when the air/fuel mixture in a combustion chamber of one or more of the cylinders fails to burn during the power stroke. When a misfire occurs, various inconveniences result, including loss of power, increased fuel consumption, excessive vibration, excessive hydrocarbon emissions and a degradation in efficiency. Furthermore, it can also result in "after burning" of unburned fuel gas in the exhaust system of the engine. This predicament can damage a catalyst of an exhaust purifying device situated in the exhaust system. Therefore, it is essential to prevent the occurrence of a misfire in an internal combustion engine.
Some efforts of detecting an engine misfire have focused on monitoring the electronics associated with the spark plugs. As an example, U.S. Pat. No. 4,886,029 to Lill et al. describes an ignition misfire detector where the ignition signal generated by the ignition coil is monitored. The ignition signal is compared to a reference signal in order to develop a control signal, which is integrated over time and which indicates a misfire condition when it exhibits a predetermined abnormal value. As another example, U.S. Pat. No. 5,215,067 to Shimasaki et al. describes a misfire detection system for detecting a value of the sparking voltage across the spark plug and for comparing this voltage with a predetermined reference. As a further example, U.S. Pat. No. 5,201,293 to Langner et al. describes a system for detecting ignition failures by monitoring the spark voltage or spark duration.
Other efforts of detecting an engine misfire have concentrated on monitoring the chemical attributes of the exhaust gas. Under normal conditions, the exhaust gas contains NOx, CO, CO.sub.2, HC, H.sub.2 O, O.sub.2, N.sub.2, and free radicals. Free radicals are gas ions that have not yet recombined to form stable molecules. The concentration of free radicals in the exhaust gas decreases as the distance from the engine exhaust valve increases. During engine misfire, the content of the exhaust changes. The exhaust after a misfire includes mostly air and fuel and only a small residual amount of combustion product gases and free radicals.
One approach for chemically monitoring engine misfire is to monitor exhaust components for sudden changes, independent of engine load and engine control parameters which can be manipulated, for instance, ignition timing, A/F ratio changes, and the like. This approach requires use of a gas analyzer for each gas component being monitored. However, use of gas analyzers is undesirable, if not impossible. First, gas analyzers are prohibitively expensive for mass use. Second, they generally require frequent maintenance. Third, they have delicate constructions and operating principles, and therefore, it would be virtually impossible to mount gas analyzers directly on an exhaust manifold. Fourth, they have slow response time. In this regard, a gas analyzer may require two seconds for generating a response, but the length of plumbing required to sample the exhaust can slow response down to over thirty seconds.
Another approach used to chemically detect misfire involves monitoring the free radicals in the exhaust. Free radicals are readily attracted by electric fields. Experiments have been conducted in which electrodes are placed in the exhaust stream. A voltage is then applied to the electrodes and the resulting current, if any, is monitored. Under normal combustion conditions, the presence of the free radicals in the exhaust is indicated by a current flow, while a misfire shows up as either a reduced current or no current. The primary problem encountered in this approach is noise. The electrodes are very susceptible to picking up electromagnetic interference (EMI). As a result, the signal-to-noise ratio of the sensor is very low. A further problem is the decreasing concentration of free radicals in the exhaust as the distance increases between the engine exhaust valve and the electrodes, as noted previously. Placement of the electrodes very close to the exhaust valve is difficult and impractical, if not impossible.